1. Field of the Invention
The present invention is related generally to load lock controls for vacuum processing chambers and more particularly to a combination differential and absolute pressure transducer for load lock control and a method of controlling load locks with such combination differential and absolute pressure transducer.
2. State of the Prior Art
Vacuum processing in reaction chambers is commonly used to deposit thin films of semiconductor materials, metal, dielectrics, and the like onto substrates in the fabrication of semiconductor devices. Typical processes that utilize such vacuum reaction chambers include chemical vapor deposition (CVD) and physical vapor deposition (PVD) and many variations of such processes, as well as etching processes to clean substrates or remove selected portions of materials. Typically, the vacuum process chamber is evacuated with a vacuum pump to a very low pressure, for example down 10xe2x88x924 torr, and, in some processes, much lower, such as 10xe2x88x926 or even 10xe2x88x927 torr. When the desired vacuum is attained, feed gases are flowed into the process chamber at desired rates and proportions to react and/or deposit desired materials onto substrate wafers. When deposition of the desired materials is complete, the wafer is removed from the process chamber and another substrate wafer is inserted into the process chamber, where the deposition process is repeated.
Significant vacuum pumping time is required to pump the process chamber down to the desired pressure, and undesirable contaminants enter the process chamber every time it is opened to atmosphere. Therefore, substantial efforts are made to avoid opening the process chamber to atmosphere and to maintain the process chamber pressure as close to the desired low deposition pressure as possible. Load locks are used, therefore, to facilitate insertion of substrates into the process chambers for deposition and/or etch processing and to remove the wafers from the process chamber while maintaining the vacuum in the process chamber.
A load lock is, essentially, a second vacuum chamber, often smaller in size than the process chamber, and connected to the process chamber by a passage with an interior xe2x80x9cdoorxe2x80x9d or large valve that can be opened for insertion and removal of the wafers into and out of the process chamber. When the interior door is closed, it seals the passage so that no air or gas can flow into or out of the process chamber through the passage. The load lock also has an exterior xe2x80x9cdoorxe2x80x9d or large valve, which opens the load lock chamber to the atmosphere to allow insertion or removal of wafers into and out of the load lock chamber. When the exterior door is closed, it seals the load lock so that no air or other gas can flow into or out of the load lock chamber.
In operation, the process chamber has its pressure maintained at the desired vacuum by a process chamber vacuum pump. With the interior door of the load lock closed, the exterior door is opened to the atmosphere, so one or more wafer substrate(s) can be inserted into the load lock chamber. With the wafer(s) in the load lock chamber, the exterior door is closed, and a load lock vacuum pump draws the air out of the load lock chamber, until the pressure in the load lock chamber is about as low as the pressure in the process chamber. Then, the interior door is opened, so the wafer substrate(s) can be moved from the load lock chamber, through the passage, and into the process chamber. When the wafer(s) are in the process chamber, the interior door can be closed while the wafer(s) are processed in the process chamber, i.e., while feed gas is fed into the process chamber and materials are either deposited on, or etched from, the wafer(s). Alternatively, but not preferably, the interior door could be left open during processing.
When the processing is complete, the wafer(s) are removed from the process chamber into the load lock chamber. The interior door is then closed to maintain the vacuum in the process chamber, while the pressure in the load lock is brought up to atmospheric pressure by allowing air or an inert gas, such as nitrogen, to flow into the load lock chamber. When the pressure in the load lock chamber is at or near atmospheric pressure, the exterior door is opened to allow removal of the processed wafer(s).
Some more complex process systems have a central transfer chamber with several process chambers branching out from the transfer chamber. In those circumstances, the load lock is usually connected by the passage and interior door to the transfer chamber.
In the past, it has been difficult to control the load lock in an efficient manner. Convection pirani pressure sensors, which have absolute pressure measuring capabilities from about 1,000 torr down to about 10xe2x88x923 torr (atmospheric pressure at sea level is about 760 torr) have been used in pressure transducers adapted to control opening of the doors in load locks. Such control of load lock doors with that type of pressure transducer has been beneficial, but problems persist. For example, the 10xe2x88x923 torr lower pressure measuring limit of the convection pirani sensors is not low enough for effective control of opening the interior door, because the process chambers are usually operated at pressures at least one to three orders of magnitude below that limit, i.e., at 10xe2x88x924 torr or even 10xe2x88x926 torr or lower. Thus, even when the load lock pressure is pumped down to 10xe2x88x923 torr, opening the interior door causes an undesirable rush of gas molecules, along with any particulate impurities and water vapor they carry along, into the process chamber. It puts a greater load on the vacuum pumps of the process and/or load lock chambers, causing larger pump down times after each opening and closing of the interior door, especially in the process chamber to get the pressure pumped back down to the desired process pressure. Such added pumping overhead adds to the processing time and decreases efficiency.
The problems are even worse on the upper pressure end, i.e., at or near atmospheric pressure (about 760 torr), because density of gas or air molecules is much greater at that pressure than at the vacuum pressures used in vacuum process chambers. Thus, opening the exterior door when pressure inside the load lock chamber is not the same as the ambient atmospheric pressure causes much stronger air currents and is much more contaminating, even when the load lock is in a clean room. Again, convection pirani sensors do have the pressure sensing capabilities in the atmospheric range, but it is impossible to set them to control exterior door opening effectively due to constantly changing ambient atmospheric pressure conditions due to weather, altitude, and the like. For example, some manufacturers set the transducer to generate a signal to open the exterior door of the load lock when pressure of the load lock chamber is brought up to 750 torr, thinking it will work for most locations that are slightly above sea level. However, ambient atmospheric pressure in Boulder, Colorado, for example, is about 630 torr, so having a transducer that opens the exterior door when pressure in the load lock chamber reaches 750 torr in Boulder, Colo., would still have adverse gas current and contamination effects. Further, ambient atmospheric pressure at any geographic location varies, such as with different weather conditions or fronts that move into and out of any particular location. Resetting such transducers to generate control signals at different pressures is not easy, may require changing software or control circuits, and is not something that is done by ordinary users.
A combination differential and absolute pressure transducer, which is the subject matter of co-pending U.S. patent application, Ser. No. 60/191,223, eliminated many of the problems described above by utilizing a differential pressure sensor for controlling operation of the exterior door and an absolute pressure sensor for controlling operation of the interior door. In that combination system, the transducer produces a signal to open the interior door to the processing chamber when the absolute pressure sensor senses that the load lock has been evacuated down to a predetermined pressure that is intended to match the evacuated pressure level of the processing chamber. On the other hand, the transducer produces a signal to open the exterior door of the load lock when the differential pressure sensor senses that the load lock chamber pressure equals the ambient atmospheric pressure.
While such combination differential and absolute pressure transducer was a significant improvement over previous load lock control systems, it still had problems. For example, modern load lock pressures reach 10xe2x88x924 torr or less, and the traditional Pirani absolute pressure sensor used in the preferred embodiment of that system is not able to provide accurate and repeatable readings in such low pressures, e.g., below about 10xe2x88x923 torr. Such traditional convection Pirani sensors also have a flat zone in a range of about 10 to 100 torr, in which accuracy is low. While a flat zone in that pressure range does not affect door control operations by the transducer, which occur at other pressures as described above, it does interfere with other pressure monitoring and control functions, such as switching from slower load lock chamber pump-down rate in high-pressure regions to faster pump-down rate n low-pressure regions. Such switching usually is set to occur at some desired set point in a range between about 0.1 torr and about 10 torr, because fast pump down at higher pressures causes turbulence that can stir up particles and contaminant wafers. Conventional Pirani sensors also do not respond as fast to pressure changes as desired for controlling such switching from slow or xe2x80x9croughingxe2x80x9d to fast or xe2x80x9cturboxe2x80x9d rates. Also, accurate readings of pressure is always important for a variety of reasons. For example, if the pressure gauge is reading high, it takes longer to reach the set point, thereby reducing through-put of products. If it reads low, it can lead to potential contamination problems.
There have also been some problems with differential pressure sensing responsiveness and accuracy due to very rapid back-filling rates used to bring the load lock pressure back up to ambient atmospheric pressure, where the exterior door is opened.
Finally, the previous combination differential and absolute pressure transducer described in co-pending U.S. patent application, Ser. No. 60/191,223, is bulky, difficult to mount, connect, and use, and is somewhat inefficient and not as reliable as desired.
Accordingly, an object of this invention is a more accurate, more reliable, more robust, better packaged, and easier to use combination differential and absolute pressure transducer for load lock control.
Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, the apparatus of the present invention may comprise, but is not limited to, a combination differential and absolute pressure transducer apparatus for controlling a load lock that facilitates transfer of parts between a room at ambient atmospheric pressure and a vacuum processing chamber maintained at a pressure less than one (1) torr and that has an evacuatable load lock chamber, an exterior door positioned between the load lock chamber and the room, an interior door positioned between the load lock chamber and the processing chamber, an exterior door actuator that is responsive to an exterior door control signal to open or close the exterior door, an interior door actuator that is responsive to an interior door control signal to open or close the interior door, and a vacuum pump connected to the load lock chamber for evacuating the load lock chamber. A slowing pump control device, such as a two-stage valve, can be installed between the vacuum pump and the load lock chamber. The combination differential and absolute pressure transducer has a differential pressure sensor that is capable of sensing a pressure difference between ambient atmospheric pressure in the room and pressure in the load lock chamber, and it has an absolute pressure sensor that is capable of sensing absolute pressure in the load lock chamber. The differential pressure sensor is mounted so that a first side of the differential pressure sensor is exposed to ambient atmospheric pressure in the room and so that a second side of the differential pressure is exposed to pressure in the load lock chamber. The absolute pressure sensor is also mounted so that it is exposed to pressure in the load lock chamber. Both the differential pressure sensor and the absolute pressure sensor can be connected in fluid flow relation to the load lock chamber by a common manifold. A differential pressure transducer circuit is connected to the differential pressure sensor and is capable of generating an exterior door control signal at a preset differential pressure value, and an absolute pressure transducer circuit is connected to the absolute pressure sensor and is capable of generating an interior door control signal at a preset absolute pressure value. An exterior door control link connected between the differential pressure transducer circuit and the exterior door is capable of delivering exterior door control signals generated by the differential pressure transducer circuit to the exterior door actuator; an interior door control link connected between the absolute pressure transducer and the interior door is capable of delivering interior door control signals generated by the absolute pressure transducer circuit to the interior door actuator. These links can be any of a variety of devices for transmitting signals, such as a wire or wires, infrared transmitter and receiver, and the like, and can include appropriate input/output components, amplifiers, and other devices as would be understood by persons skilled in the art, once they understand the principles of this invention.
The absolute pressure sensor preferably comprises a micropirani sensor with a resistivity that varies as a function of the pressure (heat exchange between a hot filament and a cooler environment) in the load lock chamber, and the absolute pressure transducer circuit can include a micropirani bridge circuit that incorporates the micropirani sensor resistive elements in the bridge circuit, which provides a signal voltage that varies as pressure in the load lock varies. A secondary temperature compensation circuit uses a resistive element on the micropirani sensor, preferably fabricated on the same substrate, but that is not exposed to load lock pressure to correct for variations in the bridge output signal that occur due to temperature changes as opposed to absolute pressure changes in the load lock. Placing this resistive element on the same substrate improves temperature compensation accuracy and response time. An analog process circuit connected to the micropirani bridge circuit conditions, amplifies, and adjusts the signal voltage from the bridge circuit for use in controlling the opening of the interior door between the load lock and the process chamber, and it includes zero and full scale adjustment features. It also produces an auxiliary output signal that is amplified even more for use especially in low pressure ranges where the regular output signal may be too weak to use accurately and dependably. A relay control circuit uses the conditioned, amplified, and adjusted voltage to generate an interior door control signal when such voltage is at a value that corresponds with a set point absolute pressure value, which can be adjusted. Hysteresis is also provided to prevent dithering and chattering of the relay at or near set point pressure.
The differential pressure sensor preferably comprises a thin film diaphragm piezo semiconductor pressure sensor in which a thin film diaphragm is positioned with the load lock chamber pressure on one side of the diaphragm and ambient atmospheric pressure of the room on another side of the diaphragm so that the diaphragm flexes one way or the other, with the direction and magnitude of such flexing dependent on the direction and magnitude of the differential pressure across the diaphragm. Resistivity of piezo semiconductor elements (preferably polysilicon resistors) varies as a function of differential pressure across a diaphragm. An analog process circuit conditions, amplifies, and adjusts the signal voltage from the bridge circuit to a more usable signal. A relay control circuit monitors the voltage from the analog process circuit and generates the exterior door control signal when the voltage of the analog process circuit corresponds with the present differential pressure value. Set point differential pressure for actuating the relay and hysteresis for preventing dithering and chattering the relay at or near set point differential pressure is also provided.
The miniaturized pressure transducer of this invention also has a very compact structure in which a manifold mounting base connects both absolute and differential pressure sensors mounted on a circuit board with the interior pressure of the load lock. Because load locks are very complex, space is usually very limited around the load lock chamber, and this miniaturized configuration is much easier to mount and less obstructive to other components and functions of the load lock.
To further achieve the foregoing and other objects, the invention may also comprise, but is not limited to, a method of automatically controlling such a load lock, including predetermining both a desired differential pressure value at which to open the external door and a desired absolute pressure value at which to open the internal door. The method then includes sensing actual differential pressure between the load lock chamber and the ambient pressure in the room, comparing the actual differential pressure to the predetermined differential pressure value, and, when the actual differential pressure equals the predetermined differential pressure value, producing and delivering an exterior door control signal to the exterior door actuator. The method also includes sensing actual absolute pressure in the load lock chamber, comparing the actual absolute pressure to the predetermined absolute pressure value, and, when the actual absolute pressure equals the predetermined absolute pressure value, producing and delivering an interior door control signal to the interior door actuator.
The method of this invention may also comprise, but is not limited to, transducing the sensed differential pressure to a voltage that is indicative of, or corresponds in value to, the sensed differential pressure, producing a differential pressure reference voltage that corresponds in value to the voltage that is transduced from the differential pressure when the differential pressure is at a desired differential pressure value for opening the exterior door, comparing the differential pressure reference voltage to such transduced voltage, and, when the transduced voltage equals the differential pressure reference voltage, producing and delivering the exterior door control signal to the exterior door actuator. This method may further include transducing the sensed absolute pressure to a voltage that is indicative of, or corresponds in value to, the absolute pressure, producing an absolute pressure reference voltage that corresponds in value to the voltage that is transduced from the absolute pressure when the absolute pressure is at a desired absolute pressure for opening the interior door, comparing the absolute pressure reference voltage to such transduced voltage, and, when the transduced voltage equals the absolute pressure reference voltage, producing and delivering the exterior door control signal to the interior door actuator. Providing hysteresis in both the absolute pressure signal and the differential pressure signal prevents dither and chattering of relays at or near set point absolute and differential pressures.
The method also includes mounting absolute and differential pressure sensors on a circuit board and mounting the circuit board on a manifold base in a manner that connects the absolute and differential pressure sensors to pressure in the interior of the load lock.